add DateTime format "yyyy-MM-dd HH:mm:ss"
optimize the QueryBuilder for "And" and "Or"

remove the "One of the queries is null" exception mechanism and the
relational unit-tests.
when some query is null, the foreach will keep continue. it's very
usefull.

related class members:
public static IMongoQuery And(IEnumerable<IMongoQuery> queries)
public static IMongoQuery Or(IEnumerable<IMongoQuery> queries)